Two U.S. lawmakers say Russia is using mobile crematoriums to hide its military involvement in eastern Ukraine.

The Kremlin promptly rejected the assertion, and it declined to comment on a report of a massive Russian military buildup near Ukraine.

The charge about the crematoriums came in an interview of House Armed Services Committee Chairman Mac Thornberry, a Republican from Texas, and Representative Seth Moulton, a Massachusetts Democrat, with Bloomberg News, excerpts of which were published Tuesday. The congressmen were part of a fact-finding mission to Ukraine last March.

"The Russians are trying to hide their casualties by taking mobile crematoriums with them,” Thornberry is quoted as saying. “They are trying to hide not only from the world, but from the Russian people, their involvement."

“Russia is clearly having a problem with their home front and the casualties they are taking from the war," Moulton is quoted as saying. "The fact that they would resort to burning the bodies of their own soldiers is horrific and shameful."

Both congressmen were careful in their comments not to disclose classified U.S. intelligence, but said they believed the reports, Bloomberg said.

The Kremlin was quick to deny the existence of mobile crematoriums. "Such allegations surely raise doubts about the sanity of those who published them, not to mention those who could have potentially made them," Russian presidential press secretary Dmitry Peskov told reporters Wednesday.

Additionally, the Kremlin refused to comment on a Western journalist's eyewitness account of Russian military moves near the Ukraine border.

A Reuters correspondent had reported seeing troops and hundreds of pieces of weaponry marshaled in the past week at a makeshift firing range 50 kilometers from the Ukraine border. The report said many of the Russian vehicles at the site had number plates and other identifying marks removed. It also said many of the troops had removed insignias from their fatigues.

Asked about the likelihood of a potential large-scale Russian invasion, Peskov referred reporters to the Defense Ministry, which said it had no comment.

NATO's military commander and its civilian chief have repeatedly accused the Kremlin of directly supporting the pro-Russian rebellion in Ukraine.

Earlier this month, NATO Secretary-General Jens Stoltenberg described the Russian military buildup on the Ukraine border as "a steady flow of heavy equipment, tanks, artillery, ammunition, air-defense systems and a lot of training." In April, he had said the buildup, which then included more than 1,000 pieces of heavy equipment, would allow pro-Russian rebels to launch new offensives against Ukraine forces in the east with little advanced warning.

Russia has consistently disputed the presence of its military personnel on Ukrainian territory. It has said those fighting alongside separatists were Russian volunteers.